What can be done to avoid
singularities when using a PTU?

To avoid singularities when using a PTU, carefully review the
use case. Mount the PTU so as to avoid looking straight up or straight
down (i.e., a tilt angle of +/- 90 degrees). The Inertial Stabilization
Module (ISM) doesn`t try to stabilize within five degrees of straight
up or down.